Geomembrane liners (often called flexible liners) are large sheets of plastic or rubber material used as a barrier to contain liquids in a surface structure. Facilities where these liners are commonly used include hazardous waste landfills and liquid impoundments, water reservoirs, and other surface impoundments.
At certain types of facilities, such as hazardous liquid surface impoundments, the geomembrane liner is often comprised of two separate layers of liner material to provide an additional margin of containment. A two-layer geomembrane liner consists of a lower plastic sheet typically made of high-density polyethylene or other polymeric material, a water-permeable intermediate material made of loosely woven or fusion-bonded plastic stringers (or, in some cases, a sand layer of several inches thickness), and an upper plastic sheet similar to the lower sheet. The grade of the bottom of the impoundment is constructed with an incline to accommodate a sump drain to collect any leakage from the impoundment through the upper liner and into the inter-liner zone.
There are no previous nonintrusive methods to indicate whether leakage is occurring through the lower liner sheet or the sump structure. However, leak problems and ground water contamination have been reported in the lower sheet of two-layer liners at some facilities and there is need for a method for detecting and locating such leaks for subsequent repair. Furthermore, those leaks which have been detected have become apparent only after substantial leakage had occurred. One embodiment of the present invention solves this difficulty by providing a method and apparatus for separately detecting and locating leaks in either the upper or the lower sheet of a two layer geomembrane liner.
Previous methods for monitoring the performance of liners after installation and use have typically been based on ground water sampling using a plurality of monitoring wells at spaced intervals along the perimeter of the impoundment. The ground water sampling method, however, provides only an indirect and delayed indication of leakage and, therefore, is not adequate for monitoring liner performance since ground water contamination may take years to occur. Furthermore, by the time a leak has been determined by this method, substantial ground water contamination may have already occurred.
Another source of inadequacy in the ground water sampling method stems from the need to have the monitoring well in the particular aquifer which is transporting the contaminants. An adequate ground water monitoring program, therefore, requires a large number of monitoring wells along the perimeter of the impoundment with a sufficient number of wells sampling water from different levels within the various aquifers beneath the impoundment. Even the most elaborate ground water monitoring system, however, cannot provide monitoring as accurate and timely as the invention system because of the inherent limitations discussed above.
One nonintrusive method for detecting and locating leaks in geomembrane liner systems uses an electrical measurement technique which takes advantage of the high electrical insulating properties of the liner with respect to the liquid contained above the liner and the soil underneath the liner. In general, geomembrane liners made from an impervious plastic material or rubber have a very high electrical resistance. A liner installed in a landfill or liquid impoundment, therefore, effectively acts as an electrical insulator between the materials contained within the facility and the surrounding environment. If the integrity of the liner is lost due to a puncture or separation, however, conductive liquid may then flow through the leak, thus establishing an electrical shunt through the liner between the contained liquid and the conductive earth in surrounding contact with the liner. The shunt is a low resistance path for current flow which forms an electrically detectable region corresponding to the a leak which may be detected and located.
The electrical measurement technique described above is discussed in greater detail in the publication "Electrical Resistivity Technique to Assess the Integrity of Geomembrane Liners," Final Technical Report, Southwest Research Institute, Project No. 14-6289, EPA Contract No. 68-03-3033 (1984), which by this reference is incorporated for all purposes.